High Resolution Finite Fault Inversions for M>4.8 Earthquakes in the 2012 Brawley Swarm Shengji Wei Acknowledgement Don Helmberger (Caltech) Rob Graves and Ken Hudnut, (USGS) Susan Owen, Eric Fielding, Zhen Liu, Jay Parker and Andrea Donnellan (JPL) Key Word: Path Calibration

Outline Overview of the 2012 Brawley swarm Path calibration from the Mw3.9 earthquake Waveform inversion of the M>5 events up to 3Hz Complementary slip distribution (triggering?) Two shallow (depth ~ 2.0km) normal earthquakes: seismological, field observation and geodetic evidence Joint inversion for the Mw4.9 normal earthquake Aseismic and seismic slip on the normal fault Conclusion

Overview Wei, et. al., 2013, GRL Seismicity from Hauksson et. al. 2013, SRL

Waveform Complexity

Ji et. al., 2002a, BSSA Finite Fault Inversion Method A simulated annealing algorithm is used to simultaneously invert for the: slip amplitude, slip direction, rise time, rupture velocity Chi-Chi Earthquake We need a layered crustal model to calculate synthetic seismogram: We need 1D velocity model(s) if not 3D.

2D horizontal cuts D=0.5km D=2.5kmD=4.5km Basin Thickness 3D velocity model: CVM-H Earthquake depth: ~5km

Path Calibration using the Mw3.9 Event Vs = (Vp – 1.36)/1.16 for Vp < 4.0km/s Brocher, 2005, BSSA frequency band: 0.05 ~ 3 Hz

Inversion results

Resolution of the data set

CVM4.0 CVMH_ PCM Model used to generate synthetic data: PCM Sensitivity for velocity structure

Normal Events

Point source double-couple moment tenor Waveform inversions at different frequency bands show similar results

Grid Search result using the M4.6 event Only a small window fits all components Ver. Rad. Tan. Vp_top Station: Hz Velocity

UAVSAR data (RPI 265) and field observation Poster: 058 (by Ken Hudnut) Static only inversion

Strong motion waveform fits and prediction (10s~4Hz) Data used

Vr=1.25km/s Vr=1.75km/s Vr=2.25km/s Rupture Velocity Enough redundancy is set for the rise time

Resolution tests

Vertical Deformation Measured by Leveling Before the swarmWith the swarm

Aseismic and seismic slip on the normal fault

Conclusion Path calibrations are needed for high resolution finite fault inversions. M5.3 and M5.4 events have complementary slip distribution. The shallow M4.9 normal earthquake has a centroid depth of ~2.0km and average rupture speed is 1.75km/s. The aseismic and seismic slip for the M4.9 normal earthquake is complementary to each other.

Thanks for your attention!

Normal Faulting

Hauksson et. al., 2013 Strike-Slip Faulting

Time table (PCM) Dep(km) Tp(s) Ts(s) pP Depth Phases

Direction Cosine U-D N-S E-W Looking Angle

LOS prediction from the slip models of M5.4 and M5.3

4 Days Seismicity

North Brawley Operations North Brawley:49.9 MW net – Currently operating at approximately 33 net MW Annual Volume of Fluid Produced:57,000 million gallons Annual Volume of Fluid Injected:53,000 million gallons 19 injection wells – Average well depth 3900 feet – Injection all < 3500 feet – Deepest injection at ~6000 feet Salinity:Average 40,000 ppm (TDS) Binary Fluid Injection Temperature:150  F – Temperature in the Injection Zone :150 to 410  F Injection Rate: Average is 525 gpm Injection Pressure:Average is 330 psig Not allowed to exceed fracture gradient at shallowest perforations 31

North Brawley Seismic Array and Wells Production wells:RED Injection wells:BLUE Seismic Stations: YELLOW 32

Synthetics test

InSAR observations

FK FD validation

Is there large amount of aseismic slip? static offset is pure prediction

Velocity Model

11369 waveform prediction for the shallow normal earthquake (M4.6)

UAVSAR 265